1. Field of the Invention
The present invention relates to a cellular communication system, and more particularly to a hand-off control method and hand-off control system for performing hand-off by giving priorities to calls.
2. Description of the Related Art
Subscribers to mobile communication systems have steadily increased in recent years, and an increase in capacity for subscribers is consequently in great demand. Reducing cell radius has been considered as one way of increasing the subscriber capacity in a mobile communication system. Reducing the cell radius, however, brings about an increase in the number of base stations in a particular area and can increase the number of channels used by subscribers.
FIG. 1a illustrates a hand-off control method in a typical mobile communication system, and FIG. 1b shows a case in which the cell radius is reduced from the case shown in FIG. 1a. 
A case is described as shown in FIG. 1a in which mobile station 110, which is present within cell 130a of base station 120a and receiving service from base station 120a, moves toward cell 130b of base station 120b. 
As mobile station 110 moves away from base station 120a, the intensity of signals from base station 120a received at mobile station 110 gradually diminishes. In this case, the intensity of received signals refers to the level of power received from the base station.
The intensity of signals from base station 120a received at mobile station 110 is periodically measured. When the measured received signal intensity falls below a predetermined threshold, a hand-off request is transmitted from mobile station 110 to base station 120a, and mobile station 110 is accordingly allowed to also receive service from base station 120b. 
The hand-off request sent from mobile station 110 to base station 120a is notified to base station 120b from base station 120a by way of a network (not shown), whereupon mobile station 110 can receive service from both base stations 120a and 120b. 
Then, as mobile station 116 moves farther away from base station 120a, the service from base station 120a is halted and mobile station 110 receives service only from base station 120b. 
The region in which mobile station 110 can receive service from both base stations 120a and 120b is area 140, which is the region in which cell 130a and cell 130b overlap.
The above-described hand-off operation is carried out with greater frequency as the cell radius is reduced as shown in FIG. 1b. Hand-off traffic thus increases as the cell radius is reduced, making forced disconnection of communication more likely to occur.
To prevent such forced disconnection, a method in which queues are maintained for requests from mobile stations requiring hand-off is disclosed in, for example, xe2x80x9cTraffic model and performance analysis for cellular mobile radio telephone systems with prioritized and nonprioritized handoff procedures xe2x80x9d by D. Hong and S. S. Rappaport (IEEE Transactions on Vehicular Technology, Vol. VT-35, 1986) and xe2x80x9cPerformance analysis of mobile cellular radio systems with priority reservation hand-off procedures xe2x80x9d by Q. A. Zeng, K. Mukumoto, and A. Fukuda (IEEE Proceedings, Vehicular Technology Conference-94, Vol. 3, 1994).
The former paper discloses a technique in which several channels dedicated to hand-off are always secured among the set channels. These secured channels are not used for new calls, and the rate of lost calls during hand-off can thus can be reduced.
The latter paper proposes adding a buffer for new call use to the technique disclosed in the former paper to provide a further reduction in the loss probability of new calls without appreciably raising the rate of disconnection of hand-off calls.
In addition, Japanese Patent Laid-open No. 264656/95 discloses a technique in which priority is assigned to a hand-off process request from a mobile station based on a calculation equation that takes into consideration the speed or the direction of movement of a moving mobile station, the hand-off process then being carried out in accordance with the priority.
Mobile stations that are moving between cells also move at various speeds, the speed of movement of mobile stations differing, for example, for a mobile station that moves by automobile and a mobile station that moves at walking speed. The permissible time interval from the request for hand-off to the completion of the hand-off process differs when the speeds of movement differ as described above, and merely carrying out the hand-off process sequentially in the order in which hand-off requests were received entails the danger of forced disconnection of a communication in mobile stations moving at high speed due to the lengthy interval from the time of hand-off request to the completion of the hand-off process.
A mobile station that is moving at high speed also passes through many cells in a prescribed time interval, and the number of requested hand-offs during a communication, as well as the possibility of forced disconnection of the communication, is therefore increased. Mobile stations that are moving between cells also have various paths of movement, with some mobile stations only moving away from the base station from which they are currently receiving service, and others maintaining a uniform distance from the base station.
The permissible time interval from the request for hand-off up to the completion of the hand-off process differs according to the various different paths of movement described above. Therefore, if the hand-off procedure is carried out merely sequentially in response to the requests from mobile stations requiring hand-off, there is the possibility that processing will be too late, resulting in the forced disconnection of communication in the case that the hand-off procedure is not carried out immediately upon request for hand-off for a mobile station that is moving away from the base station from which it is currently receiving service.
In addition, the method disclosed in Japanese Patent Laid-open No. 264656/95 suffers from the problems that calculation for giving priorities to calls is complex and, because priority is given to each call, processing is carried out frequently whenever hand-off is requested, make the processing complicated.
It is an object of the present invention to provide a hand-off control system and hand-off control method that can reduce the rate of occurrence of forced disconnection of communication by taking into consideration the permissible time interval from the request for hand-off by a mobile station until completion of the hand-off process.
To achieve the above-described object, the hand-off control system of this invention comprises: measuring means, calculating means, queue storing means, request processing means, waiting rank determining means; queue reallocating means, and queue control means.
The present invention thus constituted operates as follows: The intensity of signals received from a base station at a mobile station is periodically measured by the measuring means, and the amount of relative change for each measurement interval in the intensity of received signals measured by the measuring means is calculated by the calculating means. When hand-off is requested, the request processing means allocates the call of the mobile station that has requested hand-off to an appropriate queue of queues to which priorities are given in advance according to the amount of relative change based on the amount of relative change in the received signal intensity in the mobile station. The waiting rank determining means determines the waiting rank of a call of the mobile station within the queue in which the call of the mobile station has been allocated based on the intensity of received signals measured by the measuring means for every measurement interval of the intensity of received signals. When the amounts of relative change are updated, the queuing reallocating means periodically reallocates calls stored in each queue, for every measurement interval of the received signal intensity, such that the calls are stored in queues according to the amount of relative change in received signal intensity. The queue control means, on the other hand, monitors whether or not there are free channels within the destination cell of a moving mobile station. If there are free channels in the cell, the queue control means assigns calls allocated to the queue to free channels in an order that is based on the priority of the queue and the waiting rank within that queue.
Hand-off is thus performed in: an order that is based on the intensity of received signals and amount of relative change in the intensity of received signals at a mobile station. As a result, a mobile station for which the permissible time interval from the request for hand-off to the completion of the hand-off process is short, i.e., a mobile station that exhibits a large amount of relative change in the intensity of received signals, is processed before a mobile station for which the permissible time interval from the request for hand-off until the completion of the hand-off process is long, i.e., a mobile station that exhibits a small amount of relative change. In addition, the hand-off process for a mobile station for which the received signal intensity is weak and which is liable to be disconnected is also given higher priority.
In the hand-off control system of this invention, when the received signal intensity of a particular hand-off request call is updated, the waiting rank determining means updates the waiting rank of calls stored in each of the queues only in cases in which the received signal intensity is greater than a value obtained by adding a predetermined first hysteresis value (H1) to an upper threshold of the current waiting rank of that hand-off request call or smaller than a value obtained by subtracting the first hysteresis value (H1) from the lower threshold of the current waiting rank of the hand-off request call.
In the hand-off control system of this invention, when the amount of relative change of a particular hand-off request call is updated, the queue reallocating means reallocates calls stored in each of the queues only in cases in which the amount of relative change is greater than a value obtained by adding a predetermined second hysteresis value (H2) to the upper threshold of the class of the queue in which the hand-off request call is currently allocated or less than a value obtained by subtracting the second hysteresis value (H2) from the lower threshold of the class of queue in which the hand-off request call is currently allocated.
Thus, the use of a hysteresis characteristic in the judgment when waiting rank is determined by the waiting rank determining means and when queues are reallocated by the queue reallocating means, decreases the number of updating procedures, thereby cutting down the amount of control processing.
In an embodiment of the present invention, the amount of relative change in the intensity of received signals, when a mobile station for which the intensity of received signals was P0 at a time to moves and the intensity of received signals is P1 at time t1, is defined as:
(P1xe2x88x92P0)/(P1+P0), (P1xe2x88x92P0)/P1, (P1xe2x88x92P0)/P0 or (P1xe2x88x92P0)/(t1xe2x88x92t0)
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.